A former Republican attorney general who spoke at this summer’s GOP national convention warned that Donald Trump’s vow to imprison Hillary Clinton represents a “watershed” moment in American politics and could lead to the world perceiving the U.S. to be a “banana republic.”

Michael Mukasey, an ex-U.S. attorney general who served under President George W. Bush, told NPR Monday that he initially deemed the GOP nominee’s line at Sunday’s debate that Clinton would ‘be in jail” under a President Trump to be a quip, but had become concerned about Trump’s promise to have his attorney general appoint a special prosecutor to re-open the case on Clinton’s private email server.

“But then you have to determine whether the bringing of charges is proper,” Mukasey said, as he outlined what such a process would look like. “And then you start to get into questions of policy and whether doing that would make us look like a banana republic, which I think it would.”

Mukasey has been critical Clinton and her use of the private email server as secretary of state. But he told NPR that it would be inappropriate for a Trump administration to re-open the case after the FBI already decided not to prosecute her.

“That’s the – that to me is the – is a watershed event – that is that it’s the president of a different party. That makes it an entirely different kind of exercise in my view,” Mukasey said.

Mukasey went on to confirm that he had not endorsed either presidential candidate, while adding that he had lent advice to anyone who sought it.

He spoke at the GOP convention in July even though, back in January, he wrote in the National Review that a Trump presidency would “imperil” the country. Mukasey is a longtime associate of former New York Mayor Rudy Giuliani, a top Trump adviser, and his son works for Giuliani’s law firm.

Mukasey is not the only former GOP-appointed Department of Justice official to raise concerns about this Trump line of attack.

Paul Charlton, a U.S. attorney for Arizona under President George W. Bush, told Politico the rhetoric was “wholly inappropriate and the kind of talk more befitting a Third World country than it is our democracy.”